Nature and reactivity of Co species in a cobalt-containing beta zeolite: an FTIR study

Co-Al-BEA samples have been prepared in an attempt to introduce Co at tetra hedral lattice sites. No positive evidence of insertion has been gained, an d no new Bronsted acidity is developed. The sample, instead, contains at le ast three families of Co2+ surface sites, CoA, CoB and CoC. CoA ions, the d ominating species, are at extraframework positions (carbonyl species at 220 8 cm(-1); mononitrosyl at 1939 cm(-1): a large fraction forms dinitrosyl sp ecies). CoB and CoC are two types of Co2+ ions grafted to the surface in de fective situations (hydroxyl nests or stacking faults) and similar to Co sp ecies grafted to amorphous silica (carbonyl species at 2196 and 2184 cm(-1) ; mononitrosyl species at 1915 and 1895 cm(-1), respectively, form dinitros yl species readily). Co3+ is also present in small amounts only after NO co ntact, and is responsible for a mononitrosylic species at 1954 cm(-1). Dini trosyl species for CoA, CoB and CoC share the same modes at 1815 and 1900 c m(-1): Co3+(NO)(2) probably has the asymmetric mode at 1850 cm(-1). Interac tion with ammonia of irreversibly adsorbed NO species suggests the possible occurrence of cation pairs. The availability of two coordinative unsaturat ions on most Co species (or the occurrence of cation pairs) allows a revers ible disproportionation of NO2, consisting in the simultaneous adsorption o f two molecules with a partial electron transfer from one ligand to the oth er, with formation of NO2delta+ and NO2delta- species: at a later stage of interaction, a proper disproportionation yielding nitrate species is observ ed. The presence of Co2+ ions at extraframework positions together with som e dealumination of the samples accounts for the negligible presence of Bron sted acidity. (C) 2001 Elsevier Science B.V. All rights reserved.